Summary 1. Circulatory effects of octapressin were investigated in normal human subjects, in normotensive dogs and in dogs subjected to endotoxin or hemorrhagic shock. 2. In normotensive dogs, octapressin produced a rise in arterial pressure and a fall in cardiac output, carotid and femoral flows, with a lesser fall in renal flow. 3. In dogs subjected to endotoxin or hemorrhagic shock, octapressin produced a rise in arterial pressure but did not reverse the fall in cardiac output. A larger dose was required to produce a given pressure rise in these animals. 4. In human subjects, octapressin decreased glomerular filtration rate, renal plasma flow, and the excretion of water and electrolytes.